1. Field of the Invention
The present invention is directed to measurement of hydrophobic organic contaminants (xe2x80x9cHOCxe2x80x9d) in soil. More particularly, the present invention is directed to a method of predicting long-term desorption rates through short term measurements.
2. Background Art
Desorption rates for the release of HOCs from soils and sediments into interstitial water are typically interpreted as biphasic [1-14], with an initial rapid desorption that takes a few hours or days followed by an extremely slow desorption that can take months or years to reach an endpoint [2, 3, 12, 13, 15], and thus may result in a significant fraction sequestered in soils or sediments[2, 9, 12, 13, 15-19]. Slow desorption can be rate-limiting for biodegradation, bioremediation, and subsurface transport [13, 20-31]. Thus, it is critical for remediation efforts and application of alternative endpoints to characterize and quantify slow desorption rates.
Several analytical methods have been proposed for predicting rapidly desorbing and bioavailable fractions of sorbed HOCs. Mild solvent extraction with butanol, propanol, methanol, or ethyl acetate for short time periods ranging from 5-10 seconds [29, 46, 47] to several minutes [29, 46, 58] often parallel trends in bioavailability measured by microbial degradation or earthworm uptake. However, the solvent, dilution, agitation, and duration of extraction needed for predictive purposes will vary with each pollutant, type of microorganism, and type of soil or sediment [21, 29, 46-50], generating an impractically large matrix and highly operational results with little predictive capability. Current developments of such a matrix from methods reported in the literature are summarized in Table 1, exemplifying the arbitrary nature of solvent extraction predictions. Furthermore, addition of organic solvents could potentially swell natural organic phases in the sediments or displace HOCs from binding sites, causing a different desorption rate control compared to extraction processes occurring in natural systems [44, 51]. Finally, these quick solvent extraction techniques do not help to predict long-term desorption rates of the resistant HOC fractions, which is the limiting factor in remediation efforts.
Supercritical CO2 extractions have also been used to quantify the more easily desorbed HOC fractions versus ones that are more resistant to desorption [52]. This technique is attractive because the solvent density and diffusivity are easily adjusted with temperature and pressure, and the high diffusivities and low surface tensions accelerate HOC extraction. However, it has been shown that supercritical CO2 desorption does not parallel aqueous desorption for phenanthrene, possibly due to swelling of amorphous organic matter matrices [53]. Also, enthalpies of phenanthrene sorption to soils and systems have significantly greater magnitudes in supercritical CO2 systems than in aqueous systems, making extrapolations between the two systems inappropriate [54,55]. Furthermore, this technique does not quantitatively predict desorption rates of the more slowly desorbing, xe2x80x9cresistantxe2x80x9d HOC fractions.
The present invention provides a method for rapid assessment of long-term hydrophobic organic contaminant rates by measuring desorption by water at a plurality of elevated temperatures and pressures, determination of activation energies for desorption, and predicting, based on this data, long-term desorption at ordinary temperatures and pressures. The measurements are of relatively short duration as compared with the natural long-term desorption time frame.